In commercially-available thermal ink-jet color printers, such as the DeskJet.RTM. printer available from Hewlett-Packard Company, a color spectrum is achieved by combining yellow, magenta, and cyan inks in various proportions. The yellow, magenta, and cyan inks derive their hues from yellow, magenta, and cyan dyes, respectively. The particular set of dyes so employed constitutes a so-called "dye set".
In aqueous-based ink compositions, ink is formulated by dissolving dye in an ink vehicle. For example, a cyan ink would comprise a cyan dye dissolved in an ink vehicle. The dye molecules employed in ink-jet ink compositions are often in the form of dye salts made of a dye anion and a cation such as sodium. These dyes are designed to form solids in the target paper substrate by way of crystallization of the dye salt.
While the tendency of the dye salts to crystallize is a necessary element of ink-jet printing, it becomes problematic if the dye salt crystallizes prematurely in the ink-jet printhead about the orifice. As is known, thermal ink-jet printers employ resistors to eject droplets of ink through an orifice toward the paper substrate. During periods of pen inactivity, dye salts may crystallize in and about the orifice at its interface with the air, thereby clogging the orifice. The undesirable crystallization of dye salts about the orifice is termed "crusting" and results in the misdirection of ink droplets given a partial blockage of the orifice or even the prevention of ink droplet ejection given a complete blockage of the orifice.
It is known that the choice of cation associated with the dye anion may reduce the amount of crusting of dye salts; see, e.g., U.S. Pat. No. 4,761,180, entitled "Dyes Containing Tetramethylammonium Cation for Ink-Jet Printing Inks", assigned to the present assignee. Commercially-available anionic dye salts are often associated with such cations as sodium and ammonium, which increase the tendency of anionic dye salts to crust about the orifice. The replacement of these cations with such cations as lithium and tetramethylammonium is known to reduce crusting problems associated with certain dyes.
The choice of cation to associate with the anionic dye is all the more crucial with the recent development of inks having large amounts of inorganic salts. New generation inks employ inorganic salts to prevent the bleed of black ink into color ink. Bleed occurs as colors mix both on the surface of the paper substrate as well as within the substrate itself. It is desirable to have a border between colors that is clean and free from the invasion of one color into the other. For example, in U.S. Pat. No. 5,198,023, issued Mar. 30, 1993 and assigned to the present assignee, multivalent cations such as calcium chloride and magnesium chloride are added at concentrations ranging from about 1 to 10 wt % to yellow cationic inks to prevent bleed between yellow and black inks. The co-presence of this magnitude of inorganic salts along with certain anionic dyes, notably Direct Blue 199 ("DB 199"), precludes the association of DB 199 with undesirable cations such as sodium and ammonium, which tend to precipitate in the presence of a sufficient amount of inorganic salts.
Thus, the formulation of a specific dye set for purposes of thermal ink-jet color printing requires consideration of numerous variables. For example, the dye set employed must produce vivid, true colors of a wide spectrum when jetted onto the specially-coated paper typically employed in such color printers; see, e.g., U.S. Pat. No. 5,273,573, entitled "Specific Dye Set for Thermal Ink-Jet Printing on Paper", assigned to the present assignee. Moreover, care should be taken in the choice of cations associated with each of the dyes so that crusting about the orifice is minimized. Finally, the cation associated with an anionic dye should be suitable for use in inks having a relatively large amount of inorganic salts.
Thus, a need remains for a specific dye set that meets the above-described criteria. More particularly, a need remains to determine a cation for association with DB 199 anionic dye that will neither exhibit a tendency to precipitate in the presence of a relatively high concentration of inorganic salts nor contribute to crusting problems, all without sacrificing print quality.